1.) Field of the Invention
This invention relates to a microelectromechanical systems (MEMS) thermal device, and its method of manufacture. More particularly, this invention relates to a MEMS thermal actuator which is designed to move in a direction neither parallel to nor perpendicular to a surface of a substrate, and with motion hysteresis between the heating phase and the cooling phases of the actuator.
2) Description of Related Art
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
FIG. 1 shows an example of a prior art thermal switch, such as that described in U.S. Patent Application Publication 2004/0211178 A1. The thermal switch 10 includes two cantilevers, 100 and 200. Each cantilever 100 and 200 contains a passive beam 110 and 210, respectively, which pivot about fixed anchor points 155 and 255, respectively. A conductive circuit 120 and 220, is coupled to each passive beam 110 and 210 by a plurality of dielectric tethers 150 and 250, respectively. When a voltage is applied between terminals 130 and 140, a current is driven through conductive circuit 120. The Joule heating generated by the current causes the circuit 120 to expand relative to the unheated passive beam 110. Since the circuit is coupled to the passive beam 110 by the dielectric tether 150, the expanding conductive circuit drives the passive beam in the upward direction 165.
Applying a voltage between terminals 230 and 240 causes heat to be generated in circuit 220, which drives passive beam 210 in the direction 265 shown in FIG. 1. Therefore, one beam 100 moves in direction 165 and the other beam 200 moves in direction 265. These movements may be used to open and close a set of contacts located on contact flanges 170 and 270, each in turn located on tip members 160 and 260, respectively. The sequence of movement of contact flanges 170 and 270 on tip members 160 and 260 of switch 10 is shown in FIGS. 2a-2d, to close and open the electrical switch 10.
To begin the closing sequence, in FIG. 2a, tip member 160 and contact flange 170 are moved about 10 μm in the direction 165 by the application of a voltage between terminals 130 and 140. In FIG. 2b, tip member 260 and contact flange 270 are moved about 17 μm in the direction 265 by application of a voltage between terminals 230 and 240. This distance is required to move twice the 5 μm width of the contacts, a 4 μm initial offset between the contact flanges 170 and 270, and additional margin for tolerances of 3 μm. In FIG. 2c, tip member 160 and contact flange 170 are brought back to their initial position by removing the voltage between terminals 130 and 140. This stops current from flowing and cools the cantilever 100 and it returns to its original position. In FIG. 2d, tip member 260 and contact flange 270 are brought back to nearly their original position by removing the voltage between terminals 230 and 240. However, in this position, tip member 160 and contact flange 170 prevent tip member 260 and contact flange 270 from moving completely back to their original positions, because of the mechanical interference between contact flanges 170 and 270. In this position, contact between the faces of contact flanges 170 and 270 provides an electrical connection between cantilevers 100 and 200, such that in FIG. 2d, the electrical switch is closed. Opening the electrical switch is accomplished by reversing the movements in the steps shown in FIGS. 2a-2d. 